Resistive electrode for an electrostrictive transducer

ABSTRACT

A transducer is formed of a thin sheet of electrostrictive material having opposing faces with electrodes disposed over the opposing faces. At least one of the electrodes has a substantial resistance measurable between a pair of spaced points on the electrode.

United States Patent [1 1 Schafit [451 Jul 31,1973

[ RESISTIVE ELECTRODE FOR AN ELECTROSTRICTIVE TRANSDUCER [75] Inventor:Hugh W. Schafft, Des Plaines, Ill. [73] Assignee: Motorola, Inc.,Franklin Park, Ill.

[22] Filedz Sept. 7, 1972 [21] App]. No.: 286,883

Related U.S. Application Data [63} Continuation of Ser. No. 142,510, May12, 1971,

which is a continuation-in-part of Ser. No. 790,091, Jan. 9, 1969,abandoned.

[52] US. Cl 179/110 F, 179/110 A [51] Int. Cl H04r 17/60 [58] Field ofSearch ..179/110 A, 110 F;

INPUT SIGNAL [56] References Cited UNITED STATES PATENTS 2,305,36912/1942 Williams 179/110 A 3,059,130 10/1962 Robins BIO/9.6

Primary Examiner-Kathleen H. Claffy Assistant Examiner-Th0mas L. KundertAttorney-La Valle Ptak [57] ABSTRACT A transducer is formed of a thinsheet of electrostrictive material having opposing faces with electrodesdisposed over the opposing faces. At least one of the electrodes has asubstantial resistance measurable between a pair of spaced points on theelectrode.

8 Claims, 7 Drawing Figures PATENIEDJULM I975 7 9 55 IIIIIIIJIII I 1111/II I III] III I INPUT SIGNAL INVENTOR HUGO w SCHAFFT M Qlay ATTYS.

RESISTIVE ELECTRODE FOR AN ELECTROSTRICTIVE TRANSDUCER This is acontinuation of application Ser. No. 142,510, filed May 12, 1971, whichin turn is a continuation in part of application Ser. No. 790,091, filedJan. 9, 1969, now abandoned.

BACKGROUND OF THE INVENTION Electrostrictive transducers are formed of athin sheet of electrostrictive material having conductive electrodes onopposing faces. A signal is applied to the opposing electrodes to causemechanical movement of the transducer. Since the transducer consists ofparallel conductive plates separated by a dielectric, the load presentedby the transducer is capacitive and the transducer is frequencyresponsive. For example, a 100:1 variation in frequency of the signalapplied to the trans ducer will produce approximately a 100:1 variationin the input impedance of the transducer. This variation of inputimpedance with frequency causes a large variation in the output of thetransducer with frequency.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto provide an clectrostrictive driver having an improved frequencyresponse.

It is another object of this invention to employ frequency dependentattenuation to vary the effective amount of an electrostrictivetransducer used in operation at different frequencies.

In practicing a preferred embodiment of this invention a transducer isprovided including a thin sheet of ceramic material having opposingfaces. The transducer includes electrodes disposed on the opposing faceswith one of the electrodes being conductive and the other beingresistive relative to the conductive electrode. The resistive electrodeacts to improve the frequency response of the driver by causingincreasing attenuation of signals with increasing frequencies therebychanging the active area of response of the driver with frequency. Thetransducer may also have both electrodes formed of resistive material orthe electrodes may be formed with both resistive and conductiveportions. By mechanically loading or driving-the transducer at or nearthe point of electrical connections to the electrodes, the optimumfrequency response of the transducer is obtained.

BRIEF DESCRIPTION OF THE DRAWING The invention is illustrated by thedrawing of which:

FIG. 1 is a schematic of the equivalent circuit of a prior arttransducer;

FIG. 2 is a view of the structure of a transducer of a preferredembodiment of this invention;

FIG. 3 is a'schematic diagram of the equivalent circuit of the structureof FIG. 2;

FIGS. 4 and 5 are views of the structure of second and third embodimentsof the invention;

FIG. 6 is a sectional view of the structure of FIG. 5;

and

FIG. 7 is a view of a loudspeaker structure incorporating the featuresof the transducer structure of this invention.

DETAILED DESCRIPTION OF THE INVENTION A prior art transducer has theequivalent circuit shown in FIG. 1. Since the transducer consists ofdielectric material separating two conductive plates, the ceramictransducer presents a largely capacitive input impedance which isrepresented by capacitor 10. Since the impedance of a capacitor variesindirectly as the frequency of the signal applied thereto, the inputimpedance of the prior art driver circuit will vary in the same ratio asthe frequency of the driving signal. That is, for a 100:1 range in thefrequency of the driving signal the input impedance will vary over arange of 100:1.

An improved transducer structure using, for example ceramic material,reduces the variation of input impedance with driving frequency and isshown in FIG. 2. The structure includes a sheet of ceramic material 12having electrodes 13 and 14 on opposing faces thereof. A metal sheet 15is cemented to one side of the transducer to provide for motion in abending mode as shown. The driving signal is applied to terminals 16 and17 which are connected to the electrodes through wires 18 and 19. Oneend of the transducer is fastened to a mass 21 which prevents movementof the end of the transducer attached thereto. The other end of thetransducer is free to move and is coupled to a speaker 23. While thetransducer is shown as being coupled to speaker 23 it may, of course, becoupled to any other device which it is desired to drive.

Electrode 14 consists of two portions, a relatively resistive portion 25as compared with the electrode 13 and a relatively conductive portion26. The conductive portion '26 is at one end of the transducer and isused to make contact between the wire 19 and the resistive portion 25.

FIG. 3 shows the equivalent circuit of the structure of FIG. 2. In FIG.3 the capacitance presentedby the ceramic dielectric separating twoelectrodes, such as the electrodes 13 and 14, is a distributedcapacitance 10a combined with a resistive element 1 1. Since theimpedance presented by a resistor is linear with respect to frequencyvariation, the resistance acts to reduce the variation of the inputimpedance of the ceramic transducer with frequency. The amount ofreduction of the variation of the input impedance is determined by thevalue of the resistance of the resistor 11 (in turn determined by thecharacteristics of the electrode 25) and the dielectric constant andthickness of the ceramic which establishes the value of the distributedcapacitance 10a.

Electrode 25 may have a uniform resistivity or the resistivity may varywith position on the transducer as desired. While only electrode 14 isshown as being resistive in FIG. 2, it is, of course, possible to makeboth electrodes 13 and 14 resistive. Examples of materials found usefulfor constructing the resistive electrode 25 are graphite or a very thinlayer of deposited nickel (of the order of 0.1 mils thick). However, theresistive electrode is not limited to these materials but exhibitsahigher resistance than the resistance of the conduc-' tive electrode13.

In a structure of the type shown in FIG. 2 the input terminal 16 iscoupled to electrode 14 at the free end or radiating portion of theceramic transducer. The free portion of the transducer is moreresponsive to high frequencies than is thefixed end of the transducer.By coupling the input signal to the free end of the transducer to whichthe speaker 23 also is mechanically coupled, a further improvement ismade in the response of the transducer. Since the resistive path incombination with the parallel capacitance has a greater attenuatingeffect on high frequency signals than on low frequency signals, the highfrequency energy is concentrated in the region near the free end of thedriver and acts upon that portion of the driver only. Thus, only theportion of the driver near the free end vibrates or is active at highfrequencies. As the frequency becomes lower, more and more volume of thedriver becomes active and appears as a load across the driving source.This further acts to reduce the variation in transducer impedancepresented to the source of input signals.

The structure of FIG. 2 may be constructed in the form of a circulardisc as shown in FIG. 4. FIG. 4 shows one surface of the ceramic driverhaving conductive clectrodes 28 and 29 separated by resistive electrode30. In this structure the disc would be supported along the edges withthe center free to vibrate.

The other surface of the ceramic driver of FIG. 4 could have anelectrode arrangement of the type shown for electrodes 28, 29 and 30 orcould comprise a homogeneous conductive electrode comparable to theconductive electrode 13 shown in the embodiment of FIG. 2. Theelectrical input connections then could be made to the center electrode28 and the center of the electrode on the other side of the disc, if aresistive electrode comparable to the electrode 30 is used on that side,or to any point on the opposite side of the disc if a conductiveelectrode is used on that side of the disc. A pair of discs then can becemented together to form a bimorph transducer or a single disc could beprestressed in a dish-shaped configuration and polarized to operate as amonomorph driver.

The apex of a speaker cone such as the speaker 23 is connected to thecenter of the disc, with the vibration of the disc being restricted todifferent areas of the disc, as indicated by the dotted lines in FIG. 4,in accordance with the driving frequency applied across the electrodesat the center of the disc. For example, for input signals of the highestfrequencies, the innermost dotted ring encloses the area activelyemployed in driving the speaker attached to the center of the disc, withthe remainder of the disc remaining relatively inactive due to theattenuation of the input signals provided by the low-pass R-C filtercircuit resulting from the use of the resistive electrode 30. This R-Cfilter circuit is comparable to the one shown in FIG. 3.

As the input frequency is reduced, a greater area of the disc becomesactive in vibrating and driving a mechanical load attached to the centerof the disc, with these greater areas being indicated by the center andoutermost dotted circles in FIG. 4. For very low frequencies, the R-Cattenuation is practically zero; so that the entire mass of the discvibrates to drive a mechanical load attached to the center of the disc.

In operating a transducer of the type shown in FIG. 4, either as partofa bimorph driver or as a monomorph driver, the resistivity requiredfor the relatively resistive conductor 30 varies in accordance with theparticular type of material used for the electrostrictive element, sincethis material forms the dielectric of the capacitor formed between theelectrodes on both sides of the transducer. For electrostrictivematerials having a very high dielectric constant, the resistiveelectrode 30 can have a lower unit value of resistance than formaterials exhibiting a very low dielectric constant. In addition, thethickness of the dielectric material is an important factor. As thethickness of the material separating the electrodes on each side of thetransducer is increased,

the capacitance (such as 10a of FIG. 3) is effectively reduced; so thatthe resistance of the resistive electrode 30 would have to becorrespondingly increased to obtain the same characteristics of thelow-pass R-C filter.

A preferred material which has been used in constructing actualtransducers of the type shown in FIG. 4 is lead zirconate/Iead titanatewhich has a dielectric constant of 3,400. This dielectric constant isconsiderably higher than that of Rochelle Salt which is in the range ofIO to 200.

Referring now to FIGS. 5 and 6 there is shown a variation of thetransducer shown in FIG. 4, and the portions of the transducer shown inthe cross-section of FIG. 6 are provided with the same reference numbersused in FIG. 2 but these reference numbers are primed to indicatecomponents in the device of FIGS. 5 and 6 which are similar to thecomponents of the device shown in FIG. 2. The upper electrode 31 on theelectrostrictive transducer driving element 12' preferably is formed byelectroless plating, such as vacuum deposition or sputtering, of auniform layer of nickel over the entire upper surface of the ceramic 12'with the ceramic 12 being a lead zirconate/lead titanate ceramic ofapproximately 5 mils thickness. The resistive nickel electrode, in theform of the deposited layer, has a thickness of 0.1 mils or less.

A deposited electrode 31 is used for several reasons. First of all, thethinnest foils normally available have a thickness of 0.3 mils extendingon up to a thickness of 5 mils. Electrodes of such thicknesses made ofnickel or other similar materials exhibit too high a conductivity to beused as a resistive electrode for a practical sized transducer (of theorder of a diameter of l to 2 inches). Furthermore if a cemented foilwere used for the electrode 31, a number of air spaces would be formedbetween the foil and low spots on the contours of the ceramic surface,causing a nonuniformity in the dielectric constants between theelectrodes at different points on the electrode surfaces. To provide forthe intimate bonding necessary to eliminate such air spaces so that theelectrode 3] follows the finest contours of the ceramic surface of theelectrostrictive driving element 12', it has been found that theelectrode 31 should be a deposited film.

To increase the effective resistance between the center of the resistiveconductor 31 and its outer edge, the conductor 31 is scribed to form ahelical pattern or path from the center of the conductor 31 to itsoutermost edge. This scribing is shown by the groove 32 which is shownin FIG. 5 as forming eight turns or convolutions from the center to itsend near the outer edge of the transducer. The spiral groove 32 is cutcompletely through the resistive electrode 31 to the surface of theceramic material 12, so that the conductive path within the resistiveelectrode 31 from the center to the outer edge of the transducer isconstrained to follow the eonvolutions of the spiral. Thus, the path issubstantially extended over that which would be present if the spiralwere not formed in the electrode 31, so that the resistance from thecenter to the outer edge is substantially increased for the resistiveelectrode 31 over that which would be present without the spiral groove32.

In a transducer of the type shown in FIGS. 5 and 6 which has beensuccessfully operated, the electrostrictive material 12' was leadzirconate/lead titanate having a thickness of 5 mils and a diameter of 2inches. This electrostrictive material was prestressed and p0- larizedto form a dished transducer/diaphragm. The resistive electrode 31 wasmade of electroless deposited nickel having a thickness of approximately0.1 mils, and the conductor 13' covered the lower surface of theelectrostrictive element 12. The conductor 13' was made of highlyconductive metal and exhibited a resistance of 4 ohms thereacross. Theresistance from the center of the resistive electrode 31 to the extremeoutside edge measured 420 ohms. With a two volt input applied to theterminals 16' and 17 connected to the centers of the electrodes 13' and31, the following voltage attenuation at different frequencies measuredat the extreme circumference of the transducer was found:

The input impedance of this same device also was measured and providedthe following relationships:

1 ,400 Ohms 250 Hertz 500 Hertz 7l0 Ohms 1,000 Hertz 480 Ohms 3,000Hertz 360 Ohms 5,000 Hertz 300 Ohms From the foregoing, it can be seenthat a substantial attenuation of the input signal takes place forincreasing frequencies of this signal, which is the desired result tocause only the innermost portion of the disc to be activated foroperation at high frequencies. At the same time, it should be noted thatat the low frequency end of the applied signal range, practically thefull voltage also is applied across the electrodes at the outer edge orcircumference of the disc. Thus, substantially the entire disc isactivated at the lower frequency end of the applied frequency spectrum.Again, this is the desirable condition of operation.

With respect to the input impedance, it can be noted that the variationof the input impedance over the frequency range given in the chart isapproximately 4.7 to 1. Similar measurements made on a transducer ofcomparable dimensions, but not having the resistive electrode 3],exhibited a variation in input impedance of 20 to I over the same inputfrequency range.

By driving the transducers of FIGS. 4 or 5 and 6 at frequencies abovethe natural resonant frequency of the transducers, only a portion of thetransducers is excited, as described above; so that undesirableovertones are not generated.

A speaker system incorporating a ceramic transducer having an electrodestructure of either FIG. 4 or FIG. 5 and 6 is shown in FIG. 7. Thestructure consists of a pair of circular drivers 33 and 34 connectedtogether at their outer edges by a support member 36. The drivers 33 and34 each have a pair of ceramic sheets separated by center vanes 38 and39 respectively. The drivers also each have the electrode structure ofFIGS. 4 or 5 on both sides of the driving units. If desired, the centervanes 38 and 39 would be omitted and a single sheet of ceramic used.

The center electrode 54 of the lower driver 34 is mechanically connectedto the massof the speaker frame 40 by a connecting pillar 41 to supportthe transducer. Speaker cone 43 is mechanically fastened to the centerelectrode 51 of the upper driver 33.

Each driver 33 and 34 includes a pair of outer electrodes 45 and 46, and47 and 48 respectively and also the inner electrodes 51 and 52, and S3and 54 respectively. The outer electrodes 45-48 are electricallyconnected together and the inner elecrodes 51 and 52 of ceramic driver33 are connected to the input terminal 49. Center vanes 38 and 39 areconnected to input terminal 50.

Each of the ceramic drivers 33 and 34 have resistive electrodes 57 and60 positioned between the conductive electrodes. The resistive portionsof the electrodes cover the greatest portion of each of the opposingfaces of the ceramic drivers and most of the driving current applied tothe ceramic drivers goes through the resistive electrodes. The structureof FIG. 5 provides a speaker having a very linear response over a widefrequency range.

I claim:

1. An electrostrictive transducer including in combination, first andsecond driving elements each having inward and outward opposing faces atleast one of which has a resistive electrode thereon, said faces of saidfirst and second driving elements with said resistive electrodes furtherhaving central conductive electrodes and annular conductive electrodesalong the periphery thereof with said resistive electrodes electricallyconnecting said central and peripheral electrodes, first support meansfor holding said first and second driving elements in spaced apartparallel relationship with said inward opposing faces being adjacent,circuit means acting to electrically connect all of said peripheralelectrodes, second support means mechanically connected to the center ofsaid outward face of said first ceramic driving element, a firstelectrical input/output terminal connected to said central conductiveelectrode of said second ceramic driving element, and mechanical load/-driver means mechanically connected to the center of said outward faceof said second ceramic driving element.

2. The ceramic transducer of claim 1 wherein, said first and seconddriving elements are formed of a pair of thin ceramic sheets with aconducting center vane positioned therebetween, and both of saidopposing faces of said driving elements have resistive electrodesthereon with central conductive electrodes and annular conductiveelectrodes along the periphery thereof, a second input/output terminalconnected to each of said center vanes, and a first input terminalconnected to each of said central electrodes of said second ceramicdriving elements.

3. The ceramic transducer of claim 1 and further including a loudspeakerstructure including a frame and a speaker cone mounted thereon, thetransducer being mounted on said loudspeaker structure to furnish thedriving force therefor, said second support means being mechanicallyconnected to said frame and said speaker cone comprising said mechanicalload/driver means.-

4. A transducer for operation over a predetermined range of frequenciesincluding in combination:

a disc-shaped electrostrictive driving element formed of ceramicmaterial, the thickness of which is substantially less than the diameterand having first and second opposing faces;

a first electrode disposed on said first face, covering a major portionof the surface area thereof, and formed in a nonlinear configuration toextend the effective length of the electrically conductive pathextending from a first point located on said first face at the centerthereof and a second point located on said first face near the edgethereof; a second electrode disposed on said second face of saidelectrostrictive driving element; and means for applying electricalsignals across or for obtaining electrical signals from across saidsecond electrode and a point on said first electrode at the center ofsaid first face, the resistivity of at least said first electrode beingsufficient to operate in condjunction with distributed capacitance insaid driving element to cause the attenuation of signals at highfrequencies in said range of frequencies to be several times theattenuation of signals of lower frequencies in said range. 5. Thetransducer of claim 4 wherein, the sheeet resistance of said firstelectrode is uniform.

6. The combination according to claim 4 further including means formechancially loading or mechanically driving said transducer at a pointsubstantially adjacent the center of said first face of saidelectrostrictive driving element.

7. The combination according to claim 4 wherein said first electrode isin the form of a spiral, the center of which is located substantially atthe center of said first face of said electrostrictive driving elementso that the length of the conductive path from said first point to saidsecond point is increased by the convolutions of said spiral.

8. The combination according to claim 7 wherein said first electrodecomprises a deposited metallic electrode, with said spiral being formedby scribing through said first electrode to said electrostrictive disc,and the resistance on the first electrode between said first and secondpoints being greater by at least an order of magnitude than theresistance measured between a pair of comparably spaced points on saidsecond electrode.

1. An electrostrictive transducer including in combination, first andsecond driving elements each having inward and outward opposing faces atleast one of which has a resistive electrode thereon, said faces of saidfirst and second driving elements with said resistive electrodes furtherhaving central conductive electrodes and annular conductive electrodesalong the periphery thereof with said resistive electrodes electricallyconnecting said central and peripheral electrodes, first support meansfor holding said first and second driving elements in spaced apartparallel relationship with said inward opposing faces being adjacent,circuit means acting to electrically connect all of said peripheralelectrodes, second support means mechanically connected to the center ofsaid outward face of said first ceramic driving element, a firstelectrical input/output terminal connected to said central conductiveelectrode of said second ceramic driving element, and mechanicalload/driver means mechanically connected to the center of said outwardface Of said second ceramic driving element.
 2. The ceramic transducerof claim 1 wherein, said first and second driving elements are formed ofa pair of thin ceramic sheets with a conducting center vane positionedtherebetween, and both of said opposing faces of said driving elementshave resistive electrodes thereon with central conductive electrodes andannular conductive electrodes along the periphery thereof, a secondinput/output terminal connected to each of said center vanes, and afirst input terminal connected to each of said central electrodes ofsaid second ceramic driving elements.
 3. The ceramic transducer of claim1 and further including a loudspeaker structure including a frame and aspeaker cone mounted thereon, the transducer being mounted on saidloudspeaker structure to furnish the driving force therefor, said secondsupport means being mechanically connected to said frame and saidspeaker cone comprising said mechanical load/driver means.
 4. Atransducer for operation over a predetermined range of frequenciesincluding in combination: a disc-shaped electrostrictive driving elementformed of ceramic material, the thickness of which is substantially lessthan the diameter and having first and second opposing faces; a firstelectrode disposed on said first face, covering a major portion of thesurface area thereof, and formed in a nonlinear configuration to extendthe effective length of the electrically conductive path extending froma first point located on said first face at the center thereof and asecond point located on said first face near the edge thereof; a secondelectrode disposed on said second face of said electrostrictive drivingelement; and means for applying electrical signals across or forobtaining electrical signals from across said second electrode and apoint on said first electrode at the center of said first face, theresistivity of at least said first electrode being sufficient to operatein condjunction with distributed capacitance in said driving element tocause the attenuation of signals at high frequencies in said range offrequencies to be several times the attenuation of signals of lowerfrequencies in said range.
 5. The transducer of claim 4 wherein, thesheeet resistance of said first electrode is uniform.
 6. The combinationaccording to claim 4 further including means for mechancially loading ormechanically driving said transducer at a point substantially adjacentthe center of said first face of said electrostrictive driving element.7. The combination according to claim 4 wherein said first electrode isin the form of a spiral, the center of which is located substantially atthe center of said first face of said electrostrictive driving elementso that the length of the conductive path from said first point to saidsecond point is increased by the convolutions of said spiral.
 8. Thecombination according to claim 7 wherein said first electrode comprisesa deposited metallic electrode, with said spiral being formed byscribing through said first electrode to said electrostrictive disc, andthe resistance on the first electrode between said first and secondpoints being greater by at least an order of magnitude than theresistance measured between a pair of comparably spaced points on saidsecond electrode.